Code:
<html>
<body>

<h1>Number Guessing Game.</h1>

<?php

if (isset($_GET['secretNumber'])) {
    $secretNumber = $_GET['secretNumber'];
} else {
    $secretNumber = rand(1,20);
}

if (isset($_GET['userNumber'])) {
    $userNumber = $_GET['userNumber'];

    if ($secretNumber > $userNumber) {
        echo '<h1 style="color:blue">Your Guess is too Low.</h1>';
    } else if ($secretNumber < $userNumber) {
        echo '<h1 style="color:Red">Your Guess is too High.</h1>';
    } else {
        echo '<h1 style="color:green">You win the game.</h1>';
    }
}

?>

<form method="GET" action="">
    Guess A Number: <input name="userNumber" type="text">
    <button type="submit">Check</button>
    <input type="hidden" name="secretNumber" value="<?php echo $secretNumber; ?>">
</form>

<h2>How to Play:</h2>

<pre>
* Server will pick a secret number between 1 and 20.
* You guess what number it is.
* If your guess is too high or too low, Server will give you a hint.
* See how many turns it takes you to win!
</pre>

</body>
</html>
http://php.flashwebhost.com/mini/game.php